The present invention generally relates to the stimulation of subterranean wells and, in a representatively illustrated embodiment thereof, more particularly relates to specially designed apparatus and methods for inhibiting a screen-out condition in a subterranean well fracturing operation.
In zone fracturing of subterranean wells one previously proposed method employs a series of tubular sleeves longitudinally spaced apart along a tubular casing of an overall wellbore string. Each sleeve is slidable relative to the casing between a closed position in which the sleeve blocks associated casing side wall ports, and an open position in which the sleeve unblocks such ports to permit exit therethrough of a pressurized fracing slurry which is used to create and prop open subterranean formation fractures through which production fluid may be subsequently delivered through the wellbore string to the surface for recovery. Annular seats are secured to the sliding sleeves for movement therewith relative to the casing and are sized to sealingly receive valve actuating members, such as balls, which are successively dropped through the string. Via the use of packers or other types of seal-off structures interdigitated with the sliding sleeves, a series of fracturing zones are defined externally of the casing—each zone being associated with one of the sliding sleeves.
In carrying out a typical zone fracturing operation, with the sleeves initially in their closed positions, a ball or other type of valve actuating member is dropped through the string and caused to sealingly engage the seat portion of the lowermost sleeve. Via downward fluid pressure exerted on the dropped ball, its associated sleeve is forced in a downstream direction to its open position in which its previously covered casing ports are opened to permit pressurized frac slurry to be discharged into the formation adjacent the now-opened set of casing ports. When the fracing of this first zone is complete, a second ball is dropped into sealing engagement with the seat of the closed sliding sleeve immediately uphole of the opened first sleeve. Downward fluid pressure is then exerted on the second ball to downwardly slide its sliding sleeve and thereby open a second series of casing ports to permit pressurized fracing fluid to flow outwardly therethrough to thereby frac a second formation zone above the first fraced formation zone while the second ball isolates the fracing fluid from the first dropped ball. This sequence is repeated for each of the upwardly successive closed sliding sleeves until the zone fracturing operation is completed.
When fracing a well it is desirable to pack as much proppant into a formation as possible to keep the fractures open for production, especially close to the wellbore. A risk exists for plugging a well by packing too much proppant into a specific zone. This plugging is commonly known as a “screen-out” which may be defined as a condition arising when fracture fluids are no longer capable of carrying the proppant or the concentration of proppant becomes too great, causing the proppant to settle out in the piping and not be carried into the subterranean fractures.
A screen-out condition may cause a severe disruption in well operations and significant cost overruns due to the well known difficulties encountered in eliminating the screen-out. Various techniques have been previously proposed to prevent a screen-out condition from occurring since unplugging a screen-out is quite time consuming and expensive. Each of these known techniques carries with it problems which makes it less than entirely desirable. As but one example, a common screen-out prevention method when initiating fractures upon opening a new zone is to send fluid with no proppant therein to the formation for a period of time, and later add maximum concentrations of proppant to the fluid to place the proppant into the subterranean fractures. Due to the cost of the fluid it is desirable to minimize its use in the fracing operation. This known technique, however, substantially increases the volume of fracing fluid required, thereby materially increasing the overall cost and time needed for the fracing operation.
As can be seen from the foregoing, a need exists for improved apparatus and methods which eliminate or at least reduce the aforementioned problems created by the occurrence of screen-out conditions in well fracing operations as generally described above. It is to this need that the present invention is primarily directed.